An ozone lesson from prehistory

An extra dose of UV light is most likely to have been responsible for wiping out many land plants during the earth's largest mass extinction, 250m years ago.

Henk Visscher, of Utrecht University in the Netherlands, reports in this week's Proceedings of the National Academy of Sciences that land plants suffered a huge increase in mutation rates during the Permian mass extinction, probably caused by an excess of UV light.

Visscher looked at fossilised plant spores from the Permian period and noticed that there was a higher proportion of non-functional spores (called tetrads) than would be found in modern plants. "We also found an increase in spores that had anomalous shapes and sizes," says Visscher. Both tetrads and spores with unusual shapes and sizes are clear indicators of mutation and evidence that mutation rates were high.

The increased mutation rates are likely to be linked to an increase in UV light, resulting from a thinned ozone layer. Massive volcanos in Siberia were expelling large amounts of ozone-damaging particles (known as organohalogens) at that time. These particles were injected into the upper atmosphere, destroying significant amounts of ozone and enabling more UV light to reach the Earth's surface. "It was a similar situation to the reduced ozone layer that we see over Antarctica today," he explains. Visscher's findings show that it took land plants around 4m years to recover from this ecological crisis. In the meantime, many species became extinct and 90% of marine life was killed off.

· What did you think of this article? Mail your responses to life@guardian.co.uk and include your name and address